Nanoplasmonics relates to optical studies of tailored metallic nanostructures and their applications, and has emerged as a growing field in recent years. Nanoplasmonics allows for sub-wavelength light localization and dramatically strong local fields. Plasmons have been used to enhance linear and non-linear optical phenomena including fluorescence, surface enhanced Raman spectroscopy, and high-order harmonic generation. Proof of concept devices such as plasmon lasers, super-lenses, and metamaterials have been demonstrated.
A new generation of antennas operating at the optical and infrared frequencies is emerging from the well developed concepts in microwave antenna theory. Plasmonic nanoantennas, with their unique ability of focusing light beyond the diffraction limit, are at the core of a myriad of new exciting opportunities in photonics (Lal et al., (2007), Nat. Photonics 1:641-648; Genet C et al., (2007), Nature 455:39-46; Artar et al., (2009)). By exploiting extremely strong and localized fields in the visible wavelength range, signal enhancements of several orders of magnitude have been demonstrated in second harmonic generation (Chen et al., (1983) Phys. Rev. B 27:1965-1979.4), fluorescence (Genet C et al., (2007), Nature 455:39-46, Frey et al., (2004), Phys. Rev. Letts 93:2008015) and surface enhanced Raman scattering (SERS) (Lal et al., (2007), Nat. Photonics 1:641-648; Genet C et al., (2007), Nature 455:39-46, Kneipp et al., (1997), Phys. Rev. Letts 78:1667-1670).
The plasmonic enhancement of optical near-fields can also be extended to the infrared frequencies enabling dramatic signal enhancement in infrared (IR) spectroscopy. In analogy to SERS, this method is called surface enhanced infrared absorption (SEIRA) spectroscopy (Osawa et al., (1991) J. Physical Chemistry. 95:9914-9919; Jensen et al., (2000). Applied Spectroscopy 54:371-377; Williams et al., (2003) J. Phys. Chem. B 107:11871-11879; Enders et al., (2006) Applied Physics Letters 88:184104; Kundu et al., (2008) Chemical Physics Letters. 452:115-119; Neubrech et al., (2008) Physical Review Letters 101:157403; Bukasov et al., (2009). Analytical Chemistry 81:4531-4535; Ataka et al., (2007) Analytical Bioanalytical Chemistry 308:47-54.). Until recently, the bulk of SEIRA studies have revolved around enhancements achieved via chemically prepared or roughened metal surfaces. In these supports, however, signal enhancement factors have been limited to 10-100 range due to their random nature (Ataka et al., (2007) Analytical Bioanalytical Chemistry 308:47-54). Uncontrolled surface geometries also cause poor spectral overlap between plasmonic resonances and the molecular vibrational modes of interest. These limitations result in weaker absorption signals preventing reproducible vibrational measurements from protein monolayer films.
Additionally, advances in nanoplasmonics are critically dependent on the ability to structure metals in a controllable way at sub-100 nm resolution. The most common top-down nanopatterning techniques with high resolution are electron beam and focused ion beam lithography. Electron beam lithography (EBL) is mostly used for on-chip plasmonic nanoparticle array fabrication, while focused ion beam (FIB) tools are reserved primarily to fabricate nanoapertures in metallic films. Both EBL and FIB can offer flexibility in creating a variety of nanostructure geometries and patterns at high resolution. However, their major drawback is the low-throughput. Due to their serial nature, each nanostructure has to be created one at a time, which is both slow and expensive. In addition, for EBL, the choice of supports is also limited due to the dependence of the e-beam exposure on the support conductivity. For example, patterning on glass supports could be done by adding a conductive film (such as ITO). But, this conductive layer can interfere with the optical responses of the fabricated nanostructures. Plasmonic nanoparticle and nanowire fabrication with EBL often involves a lift-off process, which can be restrictive in creating nanostructures with high aspect ratios. While multilayer lithographic processes can be used to create nanostructures with high aspect ratios, they are cumbersome due to the involvement of multiple fabrication steps.